Radio terminals such as portable terminals have become widely used with more and more attention on a local communication among those wireless terminals. A typical such local network is an ad hoc network. An ad hoc network is a local network temporarily created as required. A wireless terminal belonging to the network does not know what terminals are in the network. Nor is there a server that centrally manages the network. The wireless terminals do not have to create a special setting for communication but autonomously recognize surrounding wireless terminals to build a network for information exchange or sharing. On this network, there is a possibility that a wireless terminal suddenly leaves the network. A need therefore exists for a mechanism that can prepare for such a change.
In building such a local network, we must take into consideration how one wireless terminal transfers data to another wireless terminal via what wireless terminal. In general, when data is transferred between computers, IP (Internet Protocol) data is transferred from an IP address to a destination IP address in the network layer of the OSI (Open System Interconnection) using a routing protocol. In this case, as a routing protocols used for path selection, there are provided a first method that employs distance vector routing in which the distance is used as the selection condition and a second method that employs link state vector routing in which the link state is used as the selection condition.
Distance vector routing, given as a first method above, uses a hop count, which is the count number of gateway hops, as a factor determining the IP-datagram delivery route in order to calculate the distance vector. The route is determined by selecting a path whose distance vector is short. The hop count refers to the number of gateways encountered by an IP-datagram from a source terminal connected to a LAN (Local Area Network) to a destination terminal connected to another LAN.
Link state vector routing, given as a second method above, maps information indicating whether or not a link between neighboring gateways is available and uses the Dijkstra algorithm as the shortest network path search algorithm. In this case, the shortest distance is calculated for one origin node to all destination nodes.
Many other routing technologies or methods have also been proposed. For example, a third method disclosed in Japanese Patent Publication Kokai JP-A No. 2000-49852 modifies distance vector routing, described above as a first method, by incrementing the hop count each time an error packet is received that is generated at the termination of the search packet signal. That is, this method varies the path selection priority by increasing the distance vector.
Another method is disclosed in Japanese Patent Publication Kokai JP-A No. 2000-341199 as a fourth method. This fourth method selects a communication path using a plurality of pre-prioritized communication path information to build a network system. If an error occurs on any of the communication paths, data to which this error information is added is sent back to the source terminal. Upon receiving the error information, the source terminal checks its own path information to switch to the next highest-priority communication path to prevent communication efficiency from decreasing.
A still another method is disclosed in Japanese Patent Publication Kokai JP-A No. 2000-4469 as a fifth method. This fifth method is applied to a wireless communication system composed of a center station, a plurality of intermediary nodes, and a plurality of wireless terminals. In this method, the center station outputs the adjustment request signal, which is received by a wireless terminal via intermediary nodes. The wireless terminal sends back quality information such as the reception field strength and a bit error rates over radio waves. The plurality of intermediary nodes, which receive the radio waves from the wireless terminal, adds the radio wave reception quality information to the received radio waves and sends them to the center station.